A major problem with heat pumps is that under normal circumstances the outdoor coil will, during the heating mode of operation, have frost accumulate thereon. As the frost thickness increases, the overall efficiency of the system decreases dramatically, and energy is wasted. Accordingly, many schemes have heretofore been proposed for detecting the frost and for taking corrective action for removing the frost from the outdoor coil. Examples of prior art systems include the following U.S. Pat. Nos. 3,170,304; 3,170,305; and 3,400,553.
Two significant factors which determine the rate of accumulation of frost on an outdoor coil during the system heating mode are the relative humidity and the temperature of the outdoor air. In accordance therewith, some prior art systems have attempted to solve the defrost problem by programming a fixed length of time of running the compressor before initiation of the defrost mode of operation based on an assumed value of outdoor air temperature. Further, such systems may be modified insofar as the selection of the number of daily defrost cycles is concerned, by the assumed value of the relative humidity of the outside air. However, such prior art systems are significantly inefficient because usually the actual outdoor temperature differs from the assumed value; further, such systems have not taken into account that the temperature of the outdoor air frequently varies significantly during the heating period. Thus, such systems provide what might be termed optimum control for only a single outdoor temperature and then only if there is no change in outdoor temperature during the defrost operation. Such systems usually do not initiate defrost at an optimum point of time; the defrost initiate function occurs either too early or too late. This causes a serious waste of energy. Also, the defrost action is frequently accomplished by reversing the main system to a "cooling" mode; this causes relatively hot refrigerant to be directed to the outdoor coil to melt the accumulated frost. However, this also causes cooling of the controlled building which is in turn typically offset by the use of high cost, lower-efficiency, electric resistance heating; again, energy is wasted.
It is an object of this invention to provide a new, significantly improved and cost effective; i.e., energy saving, defrost control system for a reverse cycle refrigeration system, i.e., heat pump.